<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .main {
      margin: 20px auto;
      width: 620px;
      border: 1px solid red;
    }

    .box {
      display: flex;
      flex-wrap: wrap;
      --gutter: 10px;
      margin: calc(0px - var(--gutter)) 0 0 calc(0px -var(--gutter));
    }

    .item {
      width: 200px;
      height: 100px;
      --gutter: 10px;
      background-color: skyblue;
      margin: var(--gutter) 0 0 var(--gutter);
    }
  </style>
</head>

<body>
  <div class="main">
    <div class="box">
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
    </div>
  </div>
</body>

</html>